THE MOLLUSC A 



the Lamellibranchs, various isolated forms of Gastropoda, and the 

 Cirrhoteuthidae among Cephalopods. The radular apparatus has 

 the form of chitinous teeth, disposed in transverse rows ; the 

 number of teeth in each row is sub- 

 ject to variation, but they are always 

 arranged symmetrically on one side 

 and the other of a central tooth 

 (Fig. 2). 



Thus there are eight lateral teeth 

 on either side of the central tooth in 

 Polyplacophora (8.1.8) ; two laterals 

 on either side of the central in 

 Scaphopoda (2.1.2); three laterals 

 on either side of the central in almost 

 all Cephalopods (3.1.3). In the 

 Gastropods the number of teeth in 

 each row varies considerably in the 

 different sub-groups. The radular 

 ribbon issues from a pharyngeal 

 caecum, in which it is secreted, and 

 is applied to the surface of paired 

 cartilaginous pieces situated on the 

 floor of the buccal cavity. These 

 so-called cartilages have a charac- 

 teristic vesicular structure quite dif- 

 ferent from that of ordinary cartilage such as is found in Cephalo- 

 pods and Opisthobranchs. Applied to these cartilaginous pieces 

 the radula, by the action of special muscles, executes backward 

 and forward rasping movements. 



The cuticular lining of the stomach is specially developed in 

 Lamellibranchs and in certain Gastropods in which it is sometimes 

 differentiated into masticatory plates provided with special muscles. 



The buccal cavity or anterior dilatation of the alimentary tube 

 receives in the Amphineura, Gastropoda, and Cephalopoda, the 

 secretion of the so-called salivary glands, of which one or several 

 pairs may be present. The oesophagus may present various 

 forms of accessory dilatations, known as "gizzards," glandular 

 regions, etc. 



The secretion of an important and voluminous digestive gland 

 is poured into the second primary dilatation of the gut or stomach : 

 this gland is called the liver, but this name must not be taken to 

 imply a physiological identity with the liver of vertebrates. The 

 liver is an acinous glandular organ, the epithelial cells of which are 

 all very similar to one another in Polyplacopho: , but in other 

 forms they are generally differentiated into ferment cells and 

 excretory cells. From the physiological point of view this gland is 



FIG. 2. 



Transverse rows of the radula. A, 

 JioivochitoH ntber ; R, Natim dou&t; L\ 

 I'ulselluin tf.tragonutn : D, Hossiii glau- 

 copis ; all much enlarged, c, central 

 tooth ; 7, lateral teeth ; m, marginal 

 teeth. (After G. (). Sars.) 



